THE KERATINIZED TISSUES 75 



Teeth are certainly homologous with the placoid scales or dermal 

 denticles of elasmobranch fish (Romer, 1955). Embryonically, the forma- 

 tion of these scales follows a similar course although the denticles are said to 

 lack an enamel layer. There are thus similarities between teeth and scales 

 which are similarly distributed as widely over the body of primitive 

 fishes as are hairs in mammals. Brandt, in particular, was led to suppose 

 that, by a degeneration of the dermal element (dentine) and by a concomit- 

 ant increase in the epidermal contribution, a horny tooth capable of 

 evolving into a hair could be produced. He found support for this in 

 the existence of the genuinely epidermal keratinous teeth of the more 

 primitive cyclostomes (see Danforth, 1932). These teeth are horny caps 

 supported by a cartilagenous pad in an everted dermal papilla. There is 

 nevertheless little resemblance to true teeth, but some to scales and other 

 epidermal thickenings found in amphibians and higher types. 



Another group of comparative morphologists has drawn attention to 

 the similarity in early embryogenesis between the hair primordia and 

 those of certain sense organs in fishes and amphibia and has suggested 

 that hairs have developed from these sense organs. A variant of this 

 theory would have that hairs descended from certain tactile spots on the 

 scales of reptiles. 



The relationship between hair groups and scales pointed out many 

 years ago by De Meijere (see Noback, 1951) is suggestive in this con- 

 nexion. Hairs are never distributed uniformly or randomly over the 

 skin; there are regional variations in density and hair-type which are 

 as much a genetically determined morphological feature as any other 

 aspects of anatomy. Even in the hair-bearing areas, the hairs are not 

 randomly distributed, but are arranged in small, well-defined groups 

 (Figs. 33 and 35). De Meijere noted that the basic group seemed to be 

 three hairs with the larger in the centre. When scales were also present 

 (Fig. 34) the hairs emerged from the underside of the scales. This concept 

 of the " basic trio group " as a morphogenetic unit has been accepted as a 

 working hypothesis by most recent workers (Carter, 1943; Carter and 

 Clarke, 1957). 



Studies of the course of embryonic appearance of hairs has partly 

 confirmed these views by showing that the first follicles to form (primary 

 follicles) are the central follicles of the trio group and that subsequent 

 follicles differentiate laterally to these to complete the group. The group 

 is not rigidly defined in numbers; some primaries remain solitary, others 

 have only a single lateral. Later other follicles, the secondaries, may 

 develop. The successive generations of follicles are related to the existence 

 in most hairy coats of the two distinct kinds of hairs : a coarser, longer 

 over-hair (guard hair) and a finer (often more woolly) under-hair. The 

 earlier developing follicles produce the over-hair and the later the fine 



